The present invention relates generally to radiographic detectors for imaging and, more particularly, to a solid-state photomultiplier (SSPM) structure with improved crosstalk and dark count rate characteristics.
Typically, in radiographic imaging systems, such as x-ray and computed tomography (CT), an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The x-ray beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated radiation received at the detector array is typically dependent upon the attenuation of the x-rays. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
As an example, conventional CT imaging systems utilize detectors that convert radiographic energy into current signals that are integrated over a time period, then measured and ultimately digitized. A drawback of such detectors however is their inability to provide data or feedback as to the number and/or energy of photons detected. During image reconstruction, data as to the number and/or energy of photons detected can be used to distinguish materials which appear identical in images reconstructed from conventional systems that do not provide this additional information. That is, conventional CT detectors have a scintillator component and a photodiode component wherein the scintillator component illuminates upon reception of radiographic energy and wherein the photodiode detects illumination of the scintillator component and provides an electrical signal as a function of the intensity of illumination. These detectors, however, are unable to provide energy discriminatory data or otherwise count the number and/or measure the energy of photons actually received by a given detector element or pixel. That is, the light output of the scintillator is proportional to the energy deposition in the scintillator, which is a function of the number of x-rays impinged as well as the energy level of each of the x-rays. Under the charge integration operation mode, the photodiode is not capable of discriminating between the energy level or the count of X-rays received by the scintillator. For example, two scintillators may illuminate with equivalent light intensity and, as such, provide equivalent output to their respective photodiodes. Yet, the number of x-rays received by each scintillator may be different as well as the x-rays' energy, but overall yield an equivalent light output.
In attempts to design scintillator-based detectors capable of X-ray photon counting and energy discrimination, detectors constructed from scintillators coupled to a solid-state photomultiplier (SSPM) have been employed. The SSPM is comprised of a plurality of Geiger-mode avalanche photodiodes (APDs) or “microcells” that amplify each single optical photon from the scintillator into a large and fast signal current pulse. When coupled with a fast scintillator material having a rapid photon decay time, SSPM based detectors can provide a photon counting, energy discriminating CT detector that does not saturate at the x-ray flux rate range typically found in conventional CT systems. An SSPM also provides a high gain with low associated noise that is highly desirable when performing photon counting and energy discrimination.
The benefits set forth above make the use of an SSPM in a detector desirable for numerous reasons; however, there are varying problems that affect current SSPM designs. Typically, SSPMs are constructed with the plurality of Geiger-mode APDs built on a single layer silicon wafer with no isolation structures positioned therebetween. Such a construction allows for optical crosstalk between APDs. That is, a small amount of light is emitted when current flows through a reverse-biased diode, as is the case when an avalanche is initiated in an APD due to the absorption of an incident photon. Because the emitted light has a peak emission of approximately 650 nm, it can travel relatively long distances in silicon before it is absorbed. When the emitted light is absorbed, it can trigger additional avalanche events, which also cause light emission. The final result can be a large number of APDs avalanching for a single photon incident on the device, thus resulting in optical crosstalk.
Conventionally, optical crosstalk between APDs is controlled by maintaining a minimum distance between adjacent APD cells. Such a mechanism for controlling optical crosstalk is only somewhat effective, and additionally, results in a loss of active area in the SSPM. That is, the spacing between adjacent APDs limits the fill factor of the active area of the SSPM and makes it difficult to design APD cell size smaller than approximately 30 microns without substantial active area loss.
Another issue with current SSPM designs is the potentially high dark count rate resulting from the whole bulk of the silicon wafer substrate. That is, a single layer silicon wafer substrate design in an SSPM can result in high diffusion dark leakage, thus adding to the overall dark count rate.
It would therefore be desirable to design a solid state photomultiplier that effectively addresses the problem of optical crosstalk between adjacent APDs. It would be further desirable to design a SSPM that reduces the dark count rate.